Movement between a surface and an imager may be tracked by 1) using the imager to acquire a series of digital images of the surface, 2) comparing successive ones of the series of digital images, and 3) analyzing differences in the successive images to quantify movement between the surface and the imager. Depending on the algorithm(s) used to analyze the differences in the successive images, it may be possible to quantify direction, velocity and other characteristics of movement.
Typically, a device in which the afore-mentioned imager is mounted will also include a light source. The light source is used to illuminate the surface with a uniform light of known wavelength. In some cases, the light source may be a light-emitting diode (LED) or laser diode. The light source is mounted such that its light is ideally reflected from the surface to the imager. However, as a result of irregularities found in most surfaces (such as contour, color and composition irregularities), different portions of a surface will typically reflect light in different amounts, and at different angles. This, in turn, leads to different amounts of light being detected by different individual pixels of the imager. Movements between the surface and the imager then lead to movements in the overall pattern of light detected by the imager's pixels, which movements are used to track movement between the surface and the imager as described in the preceding paragraph.
One problem with the above-described method and device is that, even when there is no discernable movement between the surface and the imager, surface vibrations and/or imager or system noise (collectively referred to herein as “jitter”) may lead to changes in the amount of light detected by the imager's various pixels. If suitable controls are not put in place, these changes (i.e., “jitter”) may be reported as movement. One control for eliminating the interpretation of jitter as movement is a “surface quality threshold”. Surface quality, or SQUAL, is the number of features that are found in a digital image captured by an imager. By setting a surface quality threshold, acquired images that have fewer features than the surface quality threshold can be discarded as “unusable” for tracking movement.